The latest energy efficiency standard for water heaters is NAECA III, which came into effect on Apr. 16, 2015. This standard mandates that new water heaters with capacities larger than fifty-five gallons have increased efficiency. This effectively limits the traditional and inexpensive water heater technology to capacities of fifty-five gallons or less. Water heater manufacturers are searching for a way to deliver more hot water from smaller capacity water heaters. One way to accomplish this is by storing heat in a material that that has a higher heat capacity than water, given the same volume. Such a material might be a phase change material that has a melting point near the temperature of hot water stored in domestic water heaters. An example of a phase change material is described in U.S. Pat. No. 8,887,762 to Junge et al., which is incorporated in its entirety herein. Simply encapsulating such a phase change material and locating it inside the storage tank of a water heater may improve its heat capacity but only in a limited manner for the added cost.
Most conventional water heaters draw hot water from the top of the tank and deliver incoming cold water to the bottom of the tank. Some water heaters have a cold water tap on the side near the bottom of the tank. Other water heaters have a cold water tap at the top of the tank. The latter has a cold water dip tube that carries incoming cold water from the top of the tank to the bottom of the tank. When hot water is drawn out of a water heater, the same amount of cold water enters and mixes with the rest of the stored hot water. This mixing of cold and hot water reduces the temperature of the stored water which reduces the amount of hot water that can be used. The term “draw efficiency” describes the percentage of hot water than can be drawn out of a water heater before the remaining water is considered too cold for use. Most conventional water heaters have a draw efficiency of about 70%. For example, a fifty gallon water heater with a draw efficiency of 70% can only deliver thirty-five gallons of hot water from its stored capacity without the help of its heating source.
There are several drawbacks of current technology utilizing phase change materials in water heaters. One downside is the low temperature difference between the phase change material and water due to cold water mixing with hot water before contacting the phase change material. This low temperature difference between the water and the phase change material slows down the heat transfer process. Another drawback is the added cost due to additional material and complexity in the water heater manufacturing process.